On September 6, Earth and Neptune were closest for 2018. One day later, on September 7, Neptune reaches opposition, when it is 180 degrees from the sun in our sky. In other words, on September 7, Earth passes more or less between Neptune and the sun now, as we do every year in our yearly orbit.
By closest, we don’t mean close. Neptune, the eighth planet outward from the sun, lodges in the outskirts of our solar system. Its current distance is about approximately 2.7 billion miles (4.3 billion km).
For any superior planet – that is, for any planet beyond Earth’s orbit – opposition is a special event. When any planet outside of Earth’s orbit is at or near opposition, Earth comes closest to that planet for the year, and that planet, in turn, shines most brightly in our sky. Even at opposition, however, Neptune, the eighth planet, is not bright by human standards. In fact, Neptune is the only major solar system planet that’s absolutely not visible to the unaided eye. This world is about five times fainter than the dimmest star that you can see on an inky black night. You’ll need binoculars (at least) and a detailed sky chart to see Neptune in front of the constellation Aquarius.
Because we’re more or less between Neptune and the sun around now, Neptune is rising in the east around the time of sunset, climbing highest up for the night around midnight and setting in the west around sunrise. As viewed from Earth now, this world is in front of the constellation Aquarius the Water Carrier.
In 2018, the moon turns new only two days after Neptune reaches opposition. That means dark nights to accompany this year’s Neptune opposition.
Many sky watchers will go to dark country skies to see Neptune now. They’ll find the faint stars Lambda Aquarii and Phi Aquarii with the unaided eye and then star-hop to Neptune. See sky chart below.
Even with binoculars, however, Neptune will appear only as a faint “star.”
We know it’s unlikely you’ll see Neptune unless you have optical aid and a detailed star chart.
But there are four bright planets in the September 2018 evening sky. Look for Venus and Jupiter low in the west after sunset and for Mars in the southeast. Saturn is found in the southern sky at nightfall (or as seen from the Southern Hemisphere: high overhead).
Consolation prize? Maybe! But a really good one!
By the way … want to know when Neptune next reaches perihelion, its closest point to the sun in its long (166-year) orbit? We had difficulty finding this information online. According to Jean Meeus – always an excellent source, albeit in book form – Neptune’s perihelion is coming up on September 5, 2042. Finding information on Neptune’s coming perihelion date online was harder. There’s a good starting point here. Neptune’s orbit is very nearly circular, by the way, so there’s not much difference in its perihelion/aphelion distances.
Bottom line: We’re closest to Neptune for 2018 on September 6. Neptune’s opposition – when it’s 180 degrees from the sun on the sky’s dome – is one day later, on September 7. You need optical aid to spot it. Links to charts here.